Method of forming photoresist film exhibiting uniform reflectivity through electrostatic deposition

ABSTRACT

Changing (varying, irregular) resist thickness on semiconductor wafers having irregular top surface topography or having different island sizes, affects the percent reflectance (and absorption efficiency) of incident photolithographic light, and consequently the critical dimensions of underlying features being formed (e.g., polysilicon gates). A low solvent content resist solution that can be applied as an aerosol provides a more uniform thickness resist film, eliminating or diminishing photoresist thickness variations. A top antireflective coating (TAR) also aids in uniformizing reflectance, despite resist thickness variations. The two techniques can be used alone, or together. Hence, better control over underlying gate size can be effected, without differential biasing.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 08/189,574 filedJan. 31, 1994, as a division of Ser. No. 07/907,757 filed Jun. 29, 1992,now issued as U.S. Pat. No. 5,330,883 on Jul. 19, 1994.

TECHNICAL FIELD OF THE INVENTION

The invention relates to the fabrication of integrated circuit (IC)semiconductor devices, and more particularly to exercising control overthe size of photolithographically-fabricated circuit elements,particularly polysilicon gates.

BACKGROUND OF THE INVENTION

Photolithography is a common technique employed in the manufacture ofsemiconductor devices. Typically, a semiconductor wafer is coated with alayer (film) of light-sensitive material, such as photoresist. Using apatterned mask or reticle, the wafer is exposed to projected light,typically actinic light, which manifests a photochemical effect on thephotoresist, which is subsequently chemically etched, leaving a patternof photoresist "lines" on the wafer corresponding to the pattern on themask.

This is all good in theory, until one acknowledges that the uniformityof the illuminating light varies, typically at the source of the light,and that such non-uniformity will manifest itself in the size offeatures (e.g., photoresist lines) that can be created on the wafer. Tothe end of uniformizing the light incident on and passing through themask, various techniques have been proposed, among these a techniquediscussed in commonly-owned U.S. Pat. No. 5,055,871 (Pasch).

The ultimate goal of uniformizing (homogenizing) the incident light isthat the illumination uniformity (i.e., non-uniformity) ofphotolithographic apparatus will often set a limit to how small afeature, such as a line, can be imaged in a manufacturing environment.And, as a general principle, being able to create smaller integratedcircuit features is better (faster, more compact, etc.).

Of no less concern than the ultimate size (smallness) of features, isthe ability to control the critical dimension ("cd") from one feature toanother. For example, since size generally equates with speed (e.g.,smaller is generally faster), it is disadvantageous to have onepolysilicon ("poly") gate turn out smaller (and faster) than anotherpoly gate on the same device. Conversely, it is highly desirable tofabricate all similar features (e.g., poly gates) to be the same size(i.e., with the same "cd"), especially in gate-array type devices.

Among the causes for this concern over "cd" are problems in thethickness of films overlying an irregular topography on the wafersurface. Prior to the numerous steps involved in fabricating integratedcircuit devices on a semiconductor wafer, the wafer is initially fairlyflat--exhibiting a relatively regular topography.

However, prior structure formation often leaves the top surfacetopography of the silicon wafer highly irregular, with bumps, areas ofunequal elevation, troughs, trenches and/or other surfaceirregularities. As a result of these irregularities, deposition ofsubsequent layers of materials could easily result in incompletecoverage, breaks in the deposited material, voids, etc., if it weredeposited directly over the aforementioned highly irregular surfaces. Ifthe irregularities are not alleviated at each major processing step, thetop surface topography of the surface irregularities will tend to becomeeven more irregular, causing further problems as layers stack up infurther processing of the semiconductor structure.

As mentioned above, the application and patterning of photoresist istypically a key step in the fabrication of complex integrated circuitdevices, and is a procedure that may be repeated at several differenttimes throughout the fabrication process.

It has been noticed, and is generally known, that the thickness of asubsequently applied film, particularly photoresist, will vary (in agenerally non-predictable manner) depending upon the irregulartopography of the underlying surface. (Application of an overlying filmto a flat, regular surface is generally not a problem.) For example, aphotoresist layer, even if spun-on, will exhibit a different thickness,from point-to-point over the wafer (and within the area of a givendevice) depending on the irregular topography of underlying features.

This variation in the thickness of photoresist over an irregulartopography is graphically illustrated in FIG. 1A.

FIG. 1A shows a portion of a semiconductor wafer 110 which has beenprocessed to develop raised structures 112a, 112b and 112c of FieldOxide (FOX), between which are active areas (islands) 114a and 114bhaving a lower elevation (e.g., at wafer level). The island 114a betweenthe FOX structures 112a and 112b has a width w₁ substantially smallerthan the width w₂ of the island 114b between the FOX structures 112b and112c. Having islands of different widths is not uncommon. For example,the island 114a is an "array island" having a width w₁ on the order of 3microns, and the island 114b is "I/O (Input/Output) island" having awidth w₂ on the order of hundreds of microns. Both types of islands areusually required for an integrated circuit device, and it is notuncommon to have widely varying island sizes in a single device. In anycase, the island areas are usually lower (less elevated) than the fieldoxide areas.

An overlying layer 120 of polysilicon ("poly") is deposited over thewafer, which already exhibits an irregular topography. This is accordingto known techniques, and is presented herein by way of example only.

An overlying film 130 of photoresist is applied, in any suitable manner,over the poly layer 120, and photolithographically treated to createetch-resistant "lines" (photochemically-converted areas) 132 and 134over the active areas 114a and 114b, respectively. The line features 132and 14 are shown in reverse cross-hatch from the remainder of the film130.

Ultimately, the photoresist layer 130 is etched (or washed) away,leaving only a pattern of photochemically-converted areas 132 and 134overlying the poly 120. In subsequent fabrication steps, the wafer isetched (chemical, plasma, etc.), so that all but discrete poly regions122 and 124 (shown in reverse cross-hatch) underlying respectivephotoresist features 132 and 134, respectively, are removed from thesurface of the wafer. With additional processing, not shown, the polyregions 122 and 124 may perform as gates.

Since electron flow in the lateral direction (i.e., plane of the wafer)is of primary concern in the performance of circuit elements (e.g., polygates), the transverse dimension of the poly gates 122 and 124 parallelto the plane of the wafer is of paramount interest. For purposes of thisdiscussion, this transverse dimension is termed a "critical dimension"or "cd". The poly gate 122 has a first cd, designated "cd1" and the polygate 124 has a second cd, different from the first cd, designated "cd2".

In essence, the cd's of the two poly gates are different, because thewidth of the respective overlying photoresist features is different.(Generally, the width of a poly gate will be essentially the same asthat of the overlying resist feature.)

As mentioned above, it is nearly impossible to apply a uniform layer ofphotoresist over an irregular surface. Hence, the thickness of thephotoresist 130 over the active area 114a (particularly over the areawhere the poly gate 122 is to be formed) is different than the thicknessof the photoresist 130 over the active area 114b (particularly where thepoly gate 124 is to be formed).

It is also generally known, that the reflectance of a film (such asphotoresist) will vary with its thickness. However, since the thicknessof an overlying film at any given point on the surface of thesemiconductor wafer (e.g., photoresist) is not readily determinable, thereflectance is consequently indeterminate.

This indeterminate nature of resist thickness and reflectivity overirregular underlying surfaces has important, negative ramifications inthe semiconductor fabrication process, especially in the process offabricating circuit elements having "critical dimensions".

FIG. 1B illustrates the reflectivity problem, and its manifestation inthe size of a photoresist feature. Here, in a photolithographic process,a film 140 of photoresist is exposed to light (arrows ↓↓↓↓) ofhypothetically uniform intensity. A mask 150 is interposed in the lightpath, and is provided with light-transmitting areas (lines) 152 and 154allowing light (↓↓) to impinge upon selected areas 142 and 144,respectively, of the film 140.

In FIG. 1B, the thickness of the film 140 is intentionally shown to bedifferent in the areas 142 and 144. And, as we will see, it isrelatively insignificant that the film is thicker in the area 144 thanin the area 142. And, we are ignoring, for purposes of this discussion,depth of field (depth of focus) issues that may arise from projecting amask image onto a surface of varying height.

FIG. 1C illustrates graphically the effect of film thickness (horizontalaxis) on reflectivity (i.e., the energy reflected by the film), andrelates to the issues raised in FIG. 1B. While there is a generalincrease in reflectivity with increased thickness, there is a much moreprofound (generally sinusoidal) pattern of "maxima" 170a, 170b and 170cand "minima" 172a, 172b and 172c, which exhibits that the reflectivityfor a given greater film thickness (point 172c) can well be less thanthe reflectivity for a given lesser film thickness (point 170b ).(Dashed horizontal line 174 provided as a visual aid.) Importantly,these variations are dependent on relatively small, e.g., a quarter of awavelength, variations in the thickness of the film--difficultdimensions to measure, let alone control.

Returning to FIG. 1B, it can be appreciated that it is ratherindeterminate how much of the (supposedly uniform) incident energy (↓↓↓)will be absorbed by the photoresist film, and how much will bereflected, at any given point. And, as a general proposition, the moreincident energy (↓↓↓) that is absorbed at a given point, the greater thearea of the given feature 142 or 144 will "grow". Of course, the reversewould be true for reverse masking, wherein light acts outside of thedesired feature, in which case the more light absorbed--the smaller thefeature would be.

In any case, the point is made that an irregular thickness of anoverlying film (e.g., photoresist) will impact upon the ultimatedimension of underlying features (e.g., poly gate) being formed, withcommensurate undesirable functional effects.

Certainly, if reflectivity issues were ignored, which they cannot be,the widths of all of the photoresist lines and underlying features wouldbe well-controlled. However, because the photoresist thickness variesfrom point-to-point over the wafer, and consequently its reflectivityvaries from point-to-point, the efficiency of the incident light on thephotoresist layer will vary commensurately, which will affect theultimate width of the resist features.

Evidently, the efficiency of the photolithography process is dependenton the ability of the photoresist material to absorb the radiant energy(light), and this ability is, in turn, affected by thethickness/reflectance of the photoresist.

In the prior art, it has been known to compensate approximately forknown variations (and to some extent, gross trends can be predicted) inphotoresist thicknesses by "differentially biasing" the line widths inthe high versus low reflectivity areas. And FIG. 1C illustrates that, tosome extent, one can reasonably assume that the average reflectivity foran area with greater film thickness will reflect more than an area oflesser thickness. This concept may be employed with respect torelatively large Input/Output (I/O) versus relatively small activeareas.

And, as mentioned before, in the prior art, it has also been known touse "spin-on" or other techniques in an attempt to apply a film (e.g.,photoresist) having a relatively planar top surface. Of course, arelatively planar top surface is of little help in uniformizing thethickness of a film over an underlying surface having an irregulartopography--in which case the thickness of the film would vary widelyfrom point-to-point.

It has also been known to reduce the viscosity of the photoresist sothat it goes on in a more planar manner. But, thickness will varyaccording to the topography of the underlying surface. Further, changingthe photoresist chemistry (viscosity) can have adverse side effects,such as poor photolithography resolution.

In the prior art, it has also been known to perform subsequent steps toplanarize the photoresist, which can be somewhat effective in overcomingthe reflectance issues set forth above--again, so long as thephotoresist is planarized over a relatively planar underlying surface.

Prior art techniques for accommodating "cd" variations due tophotoresist thickness variations are relatively difficult and timeconsuming to implement, and may not deliver the desired results.

The following patents, incorporated by reference herein, are cited ofgeneral interest: U.S. Pat. Nos. 4,977,330; 4,929,992; 4,912,022;4,906,852; 4,762,396; 4,698,128; 4,672,023; 4,665,007; 4,541,169;4,506,434; and 4,402,128.

DISCLOSURE OF THE INVENTION

It is a general object of the present invention to provide improvedphotolithographic (or microlithographic) techniques for the fabricationof semiconductor devices.

It is therefore an object of the present invention to provide atechnique for improving linewidth and cd (critical dimension) uniformityin photolithography (microlithography), without using spin-on techniquesand without altering photoresist chemistry.

It is another object of the present invention to provide a technique forobtaining a uniform film thickness and reflectivity of photoresistand/or other masking chemicals, regardless of the topography of anunderlying surface, particularly variations in the underlying activearea size.

According to the invention, changing (varying, irregular) resist(photoresist film) thickness resulting from resist application tosemiconductor wafers having irregular top surface topography or havingdifferent island sizes, affects the percent reflectance (and absorptionefficiency) of incident photolithographic light, and consequently thecritical dimensions of underlying features being formed (e.g.,polysilicon gates).

According to one aspect of the invention, a novel low solvent contentresist solution is formed, which can be applied (using suitable priorart application techniques) as an aerosol, to provide a more uniformthickness resist film, thereby eliminating or diminishing photoresistthickness variations.

According to another aspect of the invention, a novel top antireflectivecoating (TAR) is applied (using suitable prior art applicationtechniques) over a regular or irregular thickness photoresist film. Thisaids in uniformizing reflectance, especially with regard to photoresistfilms having thickness variations.

The two techniques of applying a more uniform film of photoresist andapplying a top antireflective coating can be used alone, or together.Hence, better control over underlying gate size can be effected, withoutdifferential biasing (differentially sizing photolithographic maskpatterns to accommodate subsequent variations in underlying featurecritical dimensions due to different size active areas, irregular topsurface wafer topography, or the like).

Other objects, features and advantages of the invention will becomeapparent in light of the following description thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a generalized, cross-sectional view of a portion of asemiconductor device (or wafer), and illustrates an exemplary problem inthe prior art, which problem is specifically addressed by the presentinvention.

FIG. 1B is a stylized cross-sectional view of a film of varyingthickness, as first discussed with respect to FIG. 1A.

FIG. 1C is a generalized graph of reflectivity (reflected energy) versusfilm thickness, and relates to FIG. 1B.

FIG. 2 is a cross-sectional, partially-perspective view of an alternateembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1A-1C have been discussed above, and illustrate the impact thatvarying film thickness, and consequent varying film reflectivity, canhave, especially in the photolithographic fabrication of polysilicongates using a patterned photoresist layer (film).

As will become evident, the inventive techniques disclosed herein do notrequire a drawing for an understanding of the subject matter sought tobe patented. 37 CFR§ 1.81 (a)

According to the invention, the problem of varying film (e.g.,photoresist) thickness exhibiting varying reflectivity is solved in oneof two ways:

1. Deposit the photoresist with a uniform thickness; or

2. Minimize reflectance variations, by using an appropriate topanti-reflective (TAR) coating.

UNIFORM DEPOSITION OF PHOTORESIST

Evidently, if a layer of photoresist, or other suitable photoreactivematerial, could be applied with uniform thickness, irrespective of theunderlying topography of the substrate, and especially in instanceswhere the underlying topography is irregular, the resulting thickness ofthe photoresist film would be relatively uniform and would exhibitrelatively uniform reflectivity. Hence, the critical dimension (cd) ofunderlying features being created with the photoresist would be moreuniform.

As mentioned hereinbefore, one of the "best" known techniques forapplying a film of photoresist is the "spin-on" technique, which strivesto create a relatively flat (planar) top surface for the photoresistfilm. However, having a flat top surface is certainly no guarantee ofhaving a uniform thickness over an irregular underlying topography. Tothe contrary, having a planar top surface photoresist film over anunderlying irregular topography, however this may be achieved, iscounter-indicative of having a uniform film thickness.

Other, non-related semiconductor fabrication processes teach thatvarious semiconductor materials can be deposited in a more-or-lessuniform layer (or film).

For example, chemical vapor deposition (CVD) of silicon nitride("nitride") creates a "blanket" nitride layer which can cover anunderlying irregular (non-planar) surface with surprising uniformity ofthickness.

By way of further example, U.S. Pat. No. 5,075,257 (Hawk, et al.),incorporated by reference herein, discloses that silicon may beaerosolized and electrostatically deposited onto various grounded, highmelting point substrates. Various preferred parameters are disclosed,relevant to aerosolizing silicon, including silicon powder size andpurity, velocity, spacing (from substrate), electrostatic charge level,temperature and time (heat cycle). The references cited in the Hawk etal. patent, are primarily directed to: other techniques of applyingsilicon, especially molten silicon;vaporizing/condensing/re-vaporizing/re-condensing solids, especiallysilicon; and electrodeposition. Each material and each process havetheir own vagaries and solutions.

According to the invention, a thicker, waxier photoresist solution isaerosolized and sprayed onto a semiconductor substrate, to form a filmof photoresist on the substrate having uniform thickness, even overareas of the substrate exhibiting non-uniform (irregular) topographies.These areas may be areas of different height, or they may be areas ofthe same height but of different width surrounded by higher or lowerfeatures. Areas of the same height but having different widths areillustrated (114a, 114b) in FIG. 1A.

Photoresist is "normally" supplied and applied in a solution ofapproximately 90% (ninety percent) casting solvent, such as ethyllactate and approximately 10% (ten percent) solids, such as novolacresin (diazoquinone). Such a solution having 90% solvent is entirelysuitable for normal spin-on and other prior art techniques of applyingphotoresist to a substrate (wafer).

According to the invention, a photoresist solution has approximately 5%(five percent) casting solvent, and 95% (ninety-five percent) solids.These are the preferred ratios. The percent solvent is in the range offrom 3% to 12%, and the percent solids is in the range of from 88% to97%. The materials are, preferably, ethyl lactate for the castingsolvent, and novolac resin (diazoquinone) for the dispersed solids.

According to the invention, the solution is created either by:

(a) simply dissolving a greater amount of solids (95% versus 10%) in thecasting solution; or

(b) preferably, starting with a "normal" solution (90% solvent) andevaporating the solvent out of the solution until the solution isapproximately 5% solvent.

The thicker, gooier, waxier, low solvent content, photoresist solutionprovided by the present invention is amenable to aerosolizing, usingprior art techniques for aerosolizing other materials. The viscosity ofthe low solvent content photoresist solution is on the order of 50-100centipoise (cp). (Water has a cp of one.) Prior art, high solventcontent, photoresist has a cp on the order of 1-10.

Heretofore, it has not been known or suggested to apply photoresist asan aerosol. Nor has it been known to provide photoresist in such a novelsolution (very low solvent content) that would be amenable to suchaerosol techniques. "Normal" (high solvent, very fluid) photoresist issimply not amenable to aerosol application techniques, nor would itappear that such has ever been contemplated.

However, the above-referenced Hawk, et al. Patent suggests that using"non-traditional" techniques ("traditional" being CVD, etc.) insemiconductor fabrication processes is gaining acceptance.

According to an alternate embodiment of the invention, photoresist isapplied in powdered form, using a xerographic-type process. Namely:

(a) evaporating the solvent from photoresist solution until itsolidifies;

(b) processing the resulting solid into powder form (using knownpowdering techniques);

(c) charging the powder; and

(d) spraying the powdered photoresist normally (at ninety degrees) ontoan electrostatically oppositely charged substrate.

TOP ANTI-REFLECTIVE (TAR) COATING

Evidently, if a layer of photoresist, or other suitable photoreactivematerial, having an irregular thickness, could be caused to exhibituniform reflectivity, the resulting line widths formed therein byphotolithography would be more uniform. Hence, the critical dimension(cd) of underlying features being created with the photoresist would bemore uniform.

According to the invention, an antireflective film is applied to theirregular top surface of photoresist over an irregular underlying layer.Being on top of the photoresist, the antireflective film is termed a"top antireflective" (TAR) film.

Antireflective films, generally, have been known since Fabry/Perot,i.e., about one hundred years, for coating lenses (optics) and the like.Generally, an antireflective film has a refractive index less than thatof the material it is coating, and has a thickness of L/4 (one quarterthe wavelength "L" of the incident light in the underlying material).The wavelength "of choice" in photolithography is 365 nanometers (nm).

Preferably, according to Fabry/Perot, the refractive index "n_(TAR) " ofthe antireflective coating would be the square root of the refractiveindex "n_(PR) ".

Common photoresist solutions (i.e., the 90% solvent solutions describedabove) have a refractive index n_(PR) of approximately 1.70, the squareroot of which is approximately 1.30. Hence, the ideal TAR would have arefractive index n_(TAR) of 1.30. Water, having a refractive index ofapproximately 1.30, would make an ideal TAR, except that it would bedifficult to work with in the semiconductor fabrication environment.Sugar also has a desirable refractive index, but would be expected tocrystallize (not desired) on top of photoresist.

International Business Machines (IBM), on the other hand, is known touse a TAR in semiconductor fabrication having a refractive index ofapproximately 1.42, which is relatively far removed from the "ideal" of1.30.

According to the present invention, a top antireflective coating isapplied over photoresist, and uniformizes reflectivity of a photoresistfilm having varying thickness.

The advantageous attributes of the inventive TAR would be that it iseasy to apply (according to known techniques) to a photoresist-coatedsemiconductor wafer, adheres well thereto, and that it has a refractiveindex very close to "ideal" (1.30). The inventor has discovered a groupof compounds satisfying these criteria. This group of compounds, one inparticular, has a refractive index relatively extremely close to theideal, namely "n_(TAR) "=1.34.

The group of compounds discovered to be nearly ideal for use as a TARover photoresist is "partially polyfluorinated compounds." While fully(versus partially) fluorinated compounds exhibit relatively low(approaching 1.30) refractive indexes, they do not adhere well tophotoresist. Partially fluorinated compounds, on the other hand, exhibitnot only advantageous (close to 1.30) refractive indexes, but also havebeen found to adhere well to photoresist.

A preferred partially fluorinated compound for use as a topantireflective coating over photoresist is:

R_(f) =F(CF₂ CF₂)₃₋₈ (R_(f) CH₂ CH₂ (O) P (O) (ONH₄)₂

This compound has been shown to exhibit the desired qualities, includinga refractive index of 1.34, which is on the order of 50% better thanIBM's TAR having a refractive index of 1.42.

An entire group of partially fluorinated compounds having low refractiveindex and easily applied and adhered to photoresist are shown on thefollowing TABLE VI, from Dow Chemical Corporation:

                  TABLE VI                                                        ______________________________________                                        Chemical Structures                                                           R.sub.f = F(CF.sub.2 CF.sub.2).sub.3-8                                        ______________________________________                                        FSA     R.sub.f CH.sub.2 CH.sub.2 SCH.sub.2 CH.sub.2 CO.sub.2 Li              FSP, FSE                                                                              (R.sub.f CH.sub.2 CH.sub.2 O)P(O)(ONH.sub.4).sub.2                            (R.sub.f CH.sub.2 CH.sub.2 O).sub.2 P(O)(ONH.sub.4)                   UR      (R.sub.f CH.sub.2 CH.sub.2 O)P(O)(OH).sub.2 (R.sub.f CH.sub.2                 CH.sub.2 O).sub.2 P(O)(OH)                                            FSN     R.sub.f CH.sub.2 CH.sub.2 O(CH.sub.2 CH.sub.2 O).sub.x H              FSN-100 R.sub.f CH.sub.2 CH.sub.2 O(CH.sub.2 CH.sub.2 O).sub.x H              FSO     R.sub.f CH.sub.2 CH.sub.2 O(CH.sub.2 CH.sub.2 O).sub.y H              FSO-100 R.sub.f CH.sub.2 CH.sub.2 O(CH.sub.2 CH.sub.2 O).sub.y H              FSC     R.sub.f CH.sub.2 CH.sub.2 SCH.sub.2 CH.sub.2 N.sup.+ (CH.sub.3).su            b.3 CH.sub.3 SO.sub.4 .sup.-                                          FSK     R.sub.f CH.sub.2 CH(OCOCH.sub.3)CH.sub.2 N.sup.+ (CH.sub.3).sub.2             CH.sub.2 CO.sub.2 .sup.-                                              TBS     R.sub.f CH.sub.2 CH.sub.2 SO.sub.3 X (X = H and                       ______________________________________                                                NH.sub.4)                                                         

The "group" of partially fluorinated compounds shown in the TABLE above,fall into the class of compounds called "surfactants". As a generalproposition, surfactants are generally employed in soap solutions andthe like. Their usefulness as a TAR over photoresist displays unexpectedresults (for compounds intended to be used as surfactants). Thecompounds can be applied in any suitable manner over the photosensitivematerial (e.g., photoresist), for example by spraying the compounds ontothe photoresist-coated wafer.

As discussed above, partially fluorinated compounds are suitable for topanti-reflective coatings over photoresist. Two issues are paramount--therefractive index, and the ability to adhere to photoresist. Generally,the less fluorinated the compound, the higher its refractive index andthe better its adhesion. Hence, a balance must be struck between thesetwo criteria.

The present invention is primarily directed to thin films ofphotoresist, for example 1-2 μm thick. Applicability to otherphotomasking materials is indicated.

By applying the resist with more uniform thickness (aerosol or powder),or by providing a top antireflective (TAR) coating, or by performingboth techniques, reflectivity is uniformized (and with TAR, isminimized), absorption of incident photolithographic light isuniformized (and increased, with TAR), and better control over featuresize, from feature to feature is effected. This provides effectivecontrol over the cd of features, especially features in differenttopographical areas (e.g., active areas versus I/O areas), andeliminates the need for differential biasing (differentially sizing themask patterns for the various features so that they turn out the samesize in the wafer, regardless of active area size). Differential biasingadds a level of complexity to design rules, which can be avoidedaccording to the present invention.

ALTERNATE EMBODIMENT

It has been discussed, above, how photoresist can be applied in aerosolform to provide a conformal layer over topological (irregular) wafers.It has also been discussed how non-conformal layers of photoresistcreate problems in subsequent fabrication steps (e.g.,photolithography), such as due to reflectance non-uniformities.

It has also been discussed how a top anti-reflective coating ofpartially fluorinated compound can be applied over photoresist.

In either case (photoresist, TAR), aerosol methods may be employed.

According to this embodiment of the invention, a coating (e.g.,photoresist, TAR) is applied to a semiconductor wafer by dipping thewafer in a reservoir containing the material to be applied, and then"pulling" the wafer out of the reservoir.

FIG. 2 shows a reservoir 202 containing a supply of material 204 to beapplied as a conformal layer over a semiconductor wafer. In thisexample, the material 204 is a high solvent content (e.g., "off theshelf") photoresist solution.

The wafer 206 has a front surface 206a containing circuit elements (notshown). The opposite, back surface of the wafer is mounted to plate 210,and held firmly against the plate by means of a vacuum supplied over aline 212. The line 212 is suitable as a mechanical support for theplate, having an orifice therethrough.

To coat the wafer, the wafer is completely immersed, edgewise, into thephotoresist supply in the reservoir. Then, it is pulled slowly out ofthe photoresist, as indicated by the arrow (↑) 214. As the wafer ispulled out of the photoresist, the photoresist will adhere to the frontsurface of the wafer, as a thin film. Excess photoresist will tend torun off the face of the wafer, due to gravity, returning to the supplyof photoresist in the reservoir. The photoresist will dry (solventevaporates) on the exposed-to-air portion of the wafer (i.e., the partthat has already been pulled out of the photoresist). By controlling thespeed at which the wafer is pulled out of the photoresist supply, thethickness of the ultimate photoresist coating over the surface of thewafer can be controlled.

The plate 210, or "wafer holder" must evidently be inert with respect tothe material contained in the reservoir. It must also seal the backsurface of the wafer from having photoresist deposited thereon. Asuitable material for the wafer holder is TEFLON. Evidently, photoresistwill tend to adhere to the wafer holder as it is pulled out of thereservoir. Therefore, the wafer holder is advantageously formed of amaterial to which the photoresist is less adhesive than with regard tothe wafer.

In the case that it is also desirable to exclude photoresist from anarrow zone around the peripheral edge of the wafer, an "exclusion band"220 is suitably disposed on the front surface of the wafer, as shown.This band may simply be a rubber ring sealing against the front surfaceof the wafer in the peripheral zone.

What is claimed is:
 1. A method of forming a photoresist film of uniformthickness over an underlying substrate to thereby form a photoresistfilm of uniform reflectivity on said substrate which comprises the stepsof:a) providing a powder of a photoresist material; b) electrostaticallycharging said powder of photoresist material at one potential; and c)electrostatically charging a substrate at an opposite potential; and d)applying a uniform thickness of said charged powder of photoresistmaterial onto said electrostatically oppositely charged substrate;tothereby form a photoresist film of uniform thickness on said substratewhich will therefore exhibit uniform reflectivity.
 2. The method ofclaim 1 wherein the step of applying said powder of photoresist ontosaid substrate further comprises spraying said powdered photoresistnormally (at ninety degrees) onto said substrate.
 3. The method of claim1 wherein the step of providing said powder of a photoresist materialfurther includes the steps of:a) evaporating the liquid from a liquidphotoresist to form a solid comprising photoresist material; and b)forming said powder from said solid photoresist material.
 4. The methodof claim 1 where said photoresist material which is formed into a powdercomprises a diazoquinone material.
 5. A method of forming a photoresistfilm of uniform thickness over an underlying semiconductor substrate tothereby form a photoresist film of uniform reflectivity on saidsemiconductor substrate which comprises the steps of:a) forming a powderof a photoresist material; b) electrostatically charging said powder ofphotoresist material at one potential; c) electrostatically charging asemiconductor substrate at an opposite potential; and d) spraying auniform thickness of said powder of photoresist material onto saidsemiconductor substrate;to thereby form a photoresist film of uniformthickness on said semiconductor substrate which will therefore exhibituniform reflectivity.